We report on the entropic properties of a stochastic radiation field. The degree of polarisation P of light, in the form of plane waves, is of the nature of an order parameter. The radiation entropy takes an analogous form to the entropy of one-dimensional Ising (two-level) spin systems in contact with a heat bath. On the basis of this analysis the degree of polarisation has a new thermodynamic significance. It is argued that within this representation, one may define an effective polarisation temperature and we show how it depends on the degree of polarisation. The results are illustrated by two examples: (i) the computation of the degree of polarisation of an incoherent mixture of partially polarised light beams and (ii) the problem of entropy production due to multiple scattering of light by a spatially random medium composed of uncorrelated and noninteracting spherical dielectric particles. Light transmitted through a multiple scattering medium is depolarised by decorrelation of the phases of the electric field components and its polarisation entropy increases. The effect of size of the spherical particles and of the optical depth on entropy production are studied numerically, using the Mie theory, via the Monte Carlo method.